This invention relates to catalyzed adsorbers for treating motor vehicle exhaust gas comprising a substrate having dispersed thereon a zeolite and an inorganic oxide washcoat, the washcoat having been impregnated with a catalytically effective amount of noble metals prior to being mixed or layered with the zeolite and optionally with a promoter. This invention further relates to a system of using such catalyzed adsorbers.
Catalytic converters containing various catalysts have been employed for years by automobile manufacturers to meet the ever-more-stringent regulations on emissions of hydrocarbons, carbon monoxide and nitrogen oxides from internal combustion engines. The continuing evolution and tightening of these regulations has made necessary the development of systems that control emission of hydrocarbons during the period immediately after start of a cold engine and before the catalytic converter normally supplied by automobile manufacturers has been sufficiently warmed by engine exhaust gas to be effective in converting hydrocarbons (often referred to as xe2x80x9ccold start conditionsxe2x80x9d). An attractive approach has been the incorporation in the exhaust system of an adsorbent capable of adsorbing hydrocarbons at the low exhaust gas temperatures associated with cold start (0 to 250xc2x0 C.) and further capable of releasing those hydrocarbons when normal exhaust system operating temperatures are achieved. Such an adsorber would normally be combined with a conventional three-way catalyst (i.e., a catalyst capable of oxidizing hydrocarbons and carbon monoxide while reducing nitrogen oxides, often referred t as a xe2x80x9cTWCxe2x80x9d) or some other non-adsorber-containing catalyst.
TWC""s of various types have been employed by themselves in the reduction of automobile emissions and have been effective in meeting the standards of the past. Ever-tightening regulations have, however, made necessary a more effective system for controlling emissions of unburnt hydrocarbons from internal combustion engines during cold start. The present invention provides such an improved system.
Various researchers have taught catalyst formulations and methods for preparation that resemble superficially the catalyzed adsorbers and method for their preparation taught here. For example, U.S. Pat. No. 4,151,121 (Gladrow) teaches a catalyst for promoting the combustion of CO in the regenerators of fluid catalytic crackers to avoid afterburning. The catalysts of that patent comprise a Group VIII element supported on an inorganic oxide and admixed with a zeolite, but also requires the presence of rhenium, chromium, manganese, or some combination thereof.
U.S. Pat. No. 5,273,945 (des Courieres et al.) also teaches catalysts for fluid catalytic cracking having metal supported on inorganic oxide binder or zeolite, but is indifferent to the nature of the metal employed and as to whether the metal is supported on the inorganic oxide or the zeolite.
In similar fashion, U.S. Pat. No. 3,833,499 (Egan et al.) teaches a catalyst for hydrocracking of hydrocarbons that employs palladium impregnated on an inorganic oxide prior to addition to a zeolite, but requires that the zeolite already be impregnated with palladium.
U.S. Pat. No. 5,278,113 (Ono et al.) teaches a catalyst for deodorization in room heating comprising a platinum group metal dispersed on alumina prior to being mixed with a zeolite. The catalyst of that invention is intended to effect the adsorption of odors from the environment in which it is deployed, periodically being heated to release and oxidatively decompose the odor components and thereby regenerating the catalyst.
More closely on point in the sense that the intended use is for treatment of motor vehicle engine exhaust gas, U.S. Pat. No. 5,354,720 (Leyer et al.) teaches a catalyst for reducing the quantity of nitrogen oxides in lean exhaust gas that comprises a first layer of aluminum or cerium oxide impregnated with a mixture of platinum and iridium and a second layer of zeolite impregnated with copper and/or iron. The present invention is concerned with reducing the amount of hydrocarbons contained in an exhaust gas stream containing a lower oxygen concentration than those taught by the Leyer patent.
U.S. Pat. No. 5,510,086 (Hemingway, et al.) teaches the use of three catalyst zones for the reduction of automobile exhaust emissions, the first of which is a conventional TWC, followed in the direction of the exhaust flow by a hydrocarbon adsorber/catalyst, which is in turn followed by another TWC. The hydrocarbon adsorber/catalyst is claimed to have a coating comprising an adhered washcoat including a zeolite and palladium. However, ""086 teaches that the noble metal is to be applied to the zeolite, thereby yielding a catalytic adsorber inferior in performance of that of the instant application.
European Patent Application No. 94309790.7 (Abe, et al.) teaches the use of one honeycomb monolith substrate which is coated in one region of the substrate with adsorbent for hydrocarbon, and in a completely different and distinct region of the substrate is coated with a catalyst layer active for three-way catalytic activity. The catalyst and adsorber in this example are not combined in any manner.
European Patent Application No. 95110617.8 (Hertl, et al) teaches a three brick system like Hemingway""s, with the middle adsorber brick containing a hole in the honeycomb monolith substrate to allow gas to bypass the adsorber. The invention further requires some sort of means to divert exhaust flow into or away from the adsorber brick.
The problems and disadvantages of the prior art are overcome and alleviated by the high-temperature aging tolerance catalyzed adsorber system for treating internal combustion engine exhaust gas. The catalyzed adsorber comprises: a substrate; a zeolite underlayer disposed over the substrate; and a catalyst overlayer disposed over the underlayer wherein the overlayer has a non-catalyst loading of about 1.0 grams per cubic inch of substrate (g/in3) or less.
Meanwhile, the method for making the catalyzed adsorber comprises: providing a substrate; disposing a zeolite underlayer over the substrate; and disposing a catalyst overlayer over the underlayer wherein the overlayer has a non-catalyst loading of about 1.0 g/in3 or less.
The above-described and other features and advantages of the present invention will be appreciated and understood by those skilled in the art from the following detailed description, drawings, and appended claims.